As users' demand for bandwidth continuously increases, a conventional copper wire broadband access system is confronted with more bandwidth bottlenecks. At the same time, an optical fiber communications technology with a massive bandwidth capacity is increasingly mature, application costs are reduced year by year, and an optical fiber access network becomes a strong competitor of a next-generation broadband access network. In particular, a passive optical network (PON) system is more competitive. FIG. 1 shows a common network structure of the PON system. The PON system generally includes one optical line terminal (OLT) that is located in a central office, one optical distribution network (ODN) that is used for splitting/coupling or multiplexing/demultiplexing, and several optical network unit (ONUs) (designated as ONU 1, ONU 2 and ONU n).
A Code Division Multiple Access (CDMA) technology is a mature wireless communications technology developed based on a branch of a digital technology, that is, a spread spectrum communications technology. A principle of the CDMA technology is based on the spread spectrum technology, that is, modulating to-be-transmitted information data with specific signal bandwidth using a high-speed code word whose bandwidth is far higher than the signal bandwidth in order to expand bandwidth of an original data signal, and then sending the signal that undergoes carrier modulation. A receive end uses completely the same code word to perform related processing on the received broadband signal, and changes the broadband signal into a narrowband signal of the original information data, that is, despreading in order to implement information communication.
In a CDMA access manner, users do not need to maintain synchronization, multiple users share an entire channel, all the users may access a same channel at any time without complex access control, and a service does not need to wait a long time for transmission. Therefore, the CDMA access manner becomes an ideal multiple access manner, and the CDMA technology makes a great success in mobile communication. In other approaches, CDMA access control is used in an optical fiber access network, and the CDMA technology is usually used in an electrical domain.
This solution has a disadvantage that because an electronic rate is limited, a rate obtained after spreading is limited, and this manner can hardly meet a requirement of development of a future high-speed image service such as a video on demand (VOD) and a high-definition television (TV) (HDTV).